1. Field of the Invention
This invention relates to a method and apparatus for adjusting read-out conditions and/or image processing conditions for a radiation image, wherein read-out conditions, under which an image signal is to be obtained, and/or image processing conditions, under which the obtained image signal is to be processed, are adjusted on the basis of an image signal representing the radiation image. This invention also relates to a radiation image analyzing method, wherein a subdivision pattern of radiation images, the shape and location of an irradiation field, an orientation in which an object was placed when the image of the object was recorded, and/or a portion of an object the image of which was recorded is determined from an image signal representing a radiation image, and a radiation image analyzing apparatus for generating characteristic measures representing the results of the determination.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal (image signal), and the image signal is processed and then used for reproducing the X-ray image as a visible image on a copy photograph, or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as the human body. A radiation image of the object is thereby stored on the stimulable phosphor sheet. The stimulable phosphor sheet is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used during the reproduction of the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT) display device, or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain the desired image density, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT display device.
In order for an image signal to be detected accurately, certain factors which affect the image signal must be set in accordance with the dose of radiation delivered to the stimulable phosphor sheet and the like. Novel radiation image recording and reproducing systems which accurately detect an image signal have been proposed. The proposed radiation image recording and reproducing systems are constituted such that a preliminary read-out operation (hereinafter simply referred to as the "preliminary readout") is carried out in order approximately to ascertain the radiation image stored on the stimulable phosphor sheet. In the preliminary readout, the stimulable phosphor sheet is scanned with a light beam having a comparatively low energy level, and a preliminary read-out image signal obtained during the preliminary readout is analyzed. Thereafter, a final read-out operation (hereinafter simply referred to as the "final readout") is carried out to obtain the image signal, which is to be used during the reproduction of a visible image. In the final readout, the stimulable phosphor sheet is scanned with a light beam having an energy level higher than the energy level of the light beam used in the preliminary readout, and the radiation image is read out with the factors affecting the image signal adjusted to appropriate values on the basis of the results of an analysis of the preliminary read-out image signal.
The term "read-out conditions" as used hereinafter means a group of various factors, which are adjustable and which affect the relationship between the amount of light emitted by the stimulable phosphor sheet during image readout and the output of a read-out means. For example, the term "read-out conditions" may refer to a read-out gain and a scale factor which define the relationship between the input to the read-out means and the output therefrom, or to the power of the stimulating rays used when the radiation image is read out.
The term "energy level of a light beam" as used herein means the level of energy of the light beam to which the stimulable phosphor sheet is exposed per unit area. In cases where the energy of the light emitted by the stimulable phosphor sheet depends on the wavelength of the irradiated light beam, i.e. the sensitivity of the stimulable phosphor sheet to the irradiated light beam depends upon the wavelength of the irradiated light beam, the term "energy level of a light beam" means the weighted energy level which is calculated by weighting the energy level of the light beam, to which the stimulable phosphor sheet is exposed per unit area, with the sensitivity of the stimulable phosphor sheet to the wavelength. In order to change the energy level of a light beam, light beams of different wavelengths may be used, the intensity of the light beam produced by a laser beam source or the like may be changed, or the intensity of the light beam may be changed by moving an ND filter or the like into and out of the optical path of the light beam. Alternatively, the diameter of the light beam may be changed in order to alter the scanning density, or the speed at which the stimulable phosphor sheet is scanned with the light beam may be changed.
Regardless of whether the preliminary readout is or is not carried out, it has also been proposed to analyze the image signal (or the preliminary read-out image signal) obtained and to adjust the image processing conditions, which are to be used when the image signal is processed, on the basis of the results of an analysis of the image signal. The term "image processing conditions" as used herein means a group of various factors, which are adjustable and set when an image signal is subjected to processing, which affects the gradation, sensitivity, or the like, of a visible image reproduced from the image signal. The proposed method is applicable to cases where an image signal is obtained from a radiation image recorded on a recording medium such as conventional X-ray film, as well as to systems using stimulable phosphor sheets.
As disclosed in, for example, U.S. Pat. No. 4,638,162 and Japanese Unexamined Patent Publication No. 61(1986)-280163, operations for calculating the values of the read-out conditions for the final readout and/or the image processing conditions are carried out by a group of algorithms which analyze an image signal (or a preliminary read-out image signal). A large number of image signals detected from a large number of radiation images are statistically processed. The algorithms which calculate the read-out conditions for the final readout and/or the image processing conditions are designed on the basis of the results obtained from this processing.
As examples of the algorithms which have heretofore been employed, methods are known wherein a probability density function of an image signal is created and analyzed, and the read-out conditions for the final readout and/or the image processing conditions are determined on the basis of the results of the analysis of the probability density function. Methods for determining the read-out conditions for the final read-out and/or the image processing conditions on the basis of the results of an analysis of the probability density function of an image signal can be classified into the following:
(1) a method as disclosed in U.S. Pat. No. 4,682,028 wherein both the maximum value and the minimum value in the range resulting in a reproduced visible image containing the necessary image information are determined from a probability density function of an image signal, and, for example, the read-out conditions for the final readout are set such that, during the final readout, the image information represented by values of the emitted light signal falling within the range of the maximum value to the minimum value is detected accurately, PA1 (2) a method as disclosed in U.S. Pat. No. 4,638,162 wherein only the maximum value is determined from a probability density function of an image signal, a value obtained by subtracting a predetermined value from the maximum value is taken as the minimum value, and the range between the maximum value and the minimum value is taken as the range resulting in a visible image containing the necessary image information, PA1 (3) a method as disclosed in Japanese Unexamined Patent Publication No. 61(1986)-280163 wherein only the minimum value is determined from a probability density function of an image signal, a value obtained by adding a predetermined value to the minimum value is taken as the maximum value, and the range between the minimum value and the maximum value is taken as the range resulting in a visible image containing the necessary image information, PA1 (4) a method as proposed in U.S. Pat. No. 4,887,305 wherein a difference probability density function is used, PA1 (5) a method as disclosed in U.S. Pat. No. 4,950,894 wherein a cumulative probability density function is used, and PA1 (6) a method as proposed in U.S. patent application Ser. No. 784,744 wherein a probability density function is divided into a plurality of small regions by using a discrimination standard. PA1 a second image signal representing the radiation image is thereafter obtained by again exposing the stimulable phosphor sheet to stimulating rays, the light emitted by the stimulable phosphor sheet being detected, and PA1 read-out conditions, under which the second image signal is to be obtained, and/or image processing conditions, under which the second image signal having been obtained is to be image processed, are adjusted on the basis of the first image signal, PA1 the method for adjusting read-out conditions and/or image processing conditions for a radiation image comprising the steps of: PA1 a second image signal representing the radiation image is thereafter obtained by again exposing the stimulable phosphor sheet to stimulating rays, the light emitted by the stimulable phosphor sheet being detected, and PA1 read-out conditions, under which the second image signal is to be obtained, and/or image processing conditions, under which the second image signal having been obtained is to be image processed, are adjusted on the basis of the first image signal, PA1 the apparatus for adjusting read-out conditions and/or image processing conditions for a radiation image comprising: PA1 the method for adjusting image processing conditions for a radiation image comprising the steps of: PA1 the apparatus for adjusting image processing conditions for a radiation image comprising: PA1 the radiation image analyzing method comprising the steps of: PA1 the radiation image analyzing apparatus comprising:
The range of an image signal resulting in a visible image containing the necessary image information is determined with one of various methods, and the read-out conditions for the final readout and/or the image processing conditions are set with respect to said range.
However, in cases where the read-out conditions for the final readout and/or the image processing conditions are calculated on the basis of the results of an analysis of the image signal in the manner described above and the image signal is detected from a recording medium, on which the irradiation field was limited during the recording of the radiation image, the radiation image cannot be ascertained accurately if the image signal is analyzed without the shape and location of the irradiation field being taken into consideration. As a result, incorrect read-out conditions and/or an incorrect image processing conditions are set, and it becomes impossible to reproduce a visible radiation image which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness.
In order to eliminate the aforesaid problem, it is necessary to determine the shape and location of an irradiation field and then to calculate the read-out conditions for the final readout and/or the image processing conditions on the basis of only the image signal representing image information stored in the region inside of the irradiation field.
The applicant has proposed various methods for determining the shape and location of an irradiation field as disclosed in, for example, U.S. Pat. Nos. 4,851,678, 4,931,644, and 4,967,079. The aforesaid problems can be eliminated by automatically determining the shape and location of the irradiation field by use of the proposed methods, and setting the read-out conditions for the final readout and/or the image processing conditions only for the region inside of the irradiation field thus found.
When radiation images are recorded on recording media, subdivision image recording is often carried out. With the subdivision image recording, the recording region on a single recording medium is divided into a plurality of predetermined subdivisions, and the respective subdivisions are exposed to radiation for image recording. The subdivision image recording is economical since, for example, when images of small object portions are recorded on large recording media, images of a plurality of object portions can be recorded on a single recording medium. Also, the speed with which radiation images are recorded and read out can be kept high.
However, in cases where irradiation fields are limited when the subdivision image recording described above is carried out on a single recording medium, the respective irradiation fields become separated from each other on the recording medium. In such cases, the shapes and locations of the irradiation fields cannot be determined accurately. A method for automatically determining the shapes and locations of a plurality of irradiation fields on a single recording medium has also been proposed. However, with the proposed method, the algorithms for determining the shapes and locations of irradiation fields become very complicated, and a very expensive apparatus is necessary for carrying out the method.
If information concerning the positions of the respective subdivisions is given by manually entering the information representing a subdivision pattern on the recording medium into an apparatus for determining the shape and location of an irradiation field when the shapes and locations of the irradiation fields are to be detected, an operation for detecting a single irradiation field in each subdivision may be carried out. Therefore, the problems in that the algorithms for determining the shapes and locations of the irradiation fields become very complicated can be eliminated. However, considerable time and labor are required to enter the information concerning the subdivision pattern each time radiation images are read out from a recording medium.
Accordingly, a need exists for a method with which a subdivision pattern of radiation images recorded on a recording medium can be determined automatically. The applicant proposed various methods for automatically determining a subdivision pattern of radiation images which have been recorded on a recording medium in, for example, Japanese Unexamined Pat. Publication Nos. 2(1990)-267679, 2(1990)-275429, 2(1990)-275432, 2(1990)-275435, 2(1990)-296235 and U.S. Pat. Nos. 4,829,181, 4,962,539, 5,028,784.
Also, in cases where the read-out conditions for the final readout and/or the image processing conditions are determined in the manner described above, it often occurs that, when radiation images of a single object were recorded on recording media with the object being placed in different orientations, the image density of a region of interest in the object varies for visible images reproduced from the radiation images.
For example, when the thoracic vertebrae of a human body are to be diagnosed, radiation images of the chest of the human body are recorded on stimulable phosphor sheets from the front and a side of the chest. In cases where the radiation image of the chest is recorded from the front of the chest, the thoracic vertebrae, which are the region of interest, overlap the mediastinum region, through which the radiation cannot pass easily. Therefore, in such cases, the amount of energy stored on part of the stimulable phosphor sheet corresponding to the thoracic vertebrae during its exposure to the radiation is comparatively small. As a result, when the stimulable phosphor sheet, on which the frontal chest image has been stored, is exposed to stimulating rays during the operation for reading out the radiation image, the part of the stimulable phosphor sheet corresponding to the thoracic vertebrae emits a comparatively small amount of light. On the other hand, in cases where the radiation image of the chest is recorded from the side of the chest, the thoracic vertebrae, which are the region of interest, overlap the lung fields, through which the radiation can pass easily. Therefore, in such cases, the amount of energy stored on part of the stimulable phosphor sheet corresponding to the thoracic vertebrae during its exposure to the radiation is comparatively large. As a result, when the stimulable phosphor sheet, on which the lateral chest image has been stored, is exposed to stimulating rays during the operation for reading out the radiation image, the part of the stimulable phosphor sheet corresponding to the thoracic vertebrae emits a comparatively large amount of light. Also, the maximum value and the minimum value of the image signal detected from the stimulable phosphor sheet do not much vary for the frontal chest image and the lateral chest image. Therefore, when the read-out conditions for the final readout and/or the image processing conditions are adjusted with conventional techniques in accordance with the maximum value and the minimum value of the image signal detected from the stimulable phosphor sheet, approximately the same values of the read-out conditions for the final readout and/or approximately the same values of the image processing conditions are set for the frontal chest image and the lateral chest image. Accordingly, if image signals are detected from the frontal chest image and the lateral chest image under the thus set read-out conditions for the final readout and visible images are reproduced from the detected image signals, and/or if the image signals detected from the frontal chest image and the lateral chest image are processed under the thus set image processing conditions and visible images are reproduced from the processed image signals, the image density of the patterns of the thoracic vertebrae in the visible image reproduced from the frontal chest image becomes comparatively low, and the image density of the patterns of the thoracic vertebrae in the visible image reproduced from the lateral chest image becomes comparatively high.
In order for the aforesaid problems to be eliminated, information concerning in what orientation the object was placed when the image of the object was recorded has heretofore been entered into an image read-out apparatus or an image processing unit each time the radiation image is read out from a stimulable phosphor sheet. The read-out conditions for the final readout and/or the image processing conditions are set in accordance with the entered information concerning the orientation in which the object was placed when the image of the object was recorded.
However, considerable time and labor are required to enter the information concerning the orientation, in which the object was placed when the image of the object was recorded, each time a radiation image is read out from a stimulable phosphor sheet. Also, it will easily occur that incorrect information concerning the orientation, in which the object was placed when the image of the object was recorded, is entered.
Therefore, a method for automatically determining the orientation, in which the object was placed when the medical image of the object was recorded on a stimulable phosphor sheet, or the like, has been proposed in, for example, U.S. Pat. No. 4,903,310.
Additionally, for the same reasons as those described above, in cases where the read-out conditions for the final readout and/or the image processing conditions are determined in the manner described above, it often occurs that the image density of a region of interest in an object varies in a reproduced visible image, depending on what portion of the object was recorded (e.g., the head, the neck, the chest, or the abdomen in cases where the object is a human body). In order for such problems to be eliminated, information concerning what portion of the object was recorded has heretofore been entered into an image read-out apparatus or an image processing unit each time the radiation image is read out from a stimulable phosphor sheet. The read-out conditions for the final readout and/or the image processing conditions are set in accordance with the entered information concerning the portion of the object the image of which was recorded.
As described above, when the read-out conditions for the final readout and/or the image processing conditions are set, it is necessary to determine a subdivision pattern of radiation images, the shape and location of an irradiation field, an orientation in which an object was placed when the image of the object was recorded, a portion of an object the image of which was recorded, or the like. A correction should then be made in accordance with the results of the determination. Thereafter, appropriate read-out conditions for the final readout and/or appropriate image processing conditions should be set.
Recently, a method for utilizing a neural network has been proposed and is being used in various fields.
The neural network is provided with a learning function by a back propagation method. Specifically, when information (an instructor signal), which represents whether an output signal obtained when an input signal is given is or is not correct, is fed into the neural network, the weights of connections between units in the neural network (i.e. the weights of synapse connections) are corrected. By repeating the learning operation of the neural network, the probability that a correct answer will be obtained in response to a new input signal can be kept high.
By feeding an image signal representing a radiation image into the neural network, the read-out conditions for the final readout and/or the image processing conditions described above can be set with the neural network. Also, the determination of a subdivision pattern of radiation images, the shape and location of an irradiation field, an orientation in which an object was placed when the image of the object was recorded, a portion of an object the image of which was recorded, or the like, can be carried out with the neural network. A correction can then be made in accordance with the results of the determination. Thereafter, appropriate read-out conditions for the final readout and/or appropriate image processing conditions can be set.
Specifically, an image signal representing a radiation image is fed into the neural network. From the neural network, outputs representing characteristic measures, which indicate the read-out conditions for the final readout, the image processing conditions, and/or the results of the determination of various items, are obtained. By repeating the learning operation of the neural network, characteristic measures more accurately representing the read-out conditions for the final readout, the image processing conditions, and/or the results of the determination of various items can be obtained.
However, with the aforesaid method for analyzing the probability density function of an image signal and adjusting the read-out conditions for the final readout and/or the image processing conditions on the basis of the results of the analysis of the probability density function, various characteristic values are calculated by carrying out a threshold value process and a local analysis of the probability density function. Therefore, too much importance is attached to local features of the probability density function, and incorrect results are often obtained.
Also, with the above-described method for adjusting the read-out conditions for the final readout and/or the image processing conditions by utilizing a neural network, a local analysis is not carried out, and therefore the problems can be prevented from occurring in that too much importance is attached to local features and incorrect results are often obtained. However, an image signal is directly fed into the neural network, and the learning operation of the neural network is carried out repeatedly. Therefore, a very long period of time is required for the learning operation. This is not advantageous in practice.